Here's a shortened description of Parallax from the official film synopsis:
A new enemy called Parallax threatens to destroy the balance of power in the Universe. Their [the Green Lantern Corps] fate and the fate of Earth lie in the hands of their newest recruit, the first human ever selected: Hal Jordan.
The picture above reportedly comes from an upcoming issue of Green Lantern, possibly issue #60 or #61. Not sure how it could have leaked, but if the issues have already been sent to press, it's possible the image comes from a screen shot of the digital files, or perhaps a copy of the comic proof.
However the image got out, it only raises questions. Will Parallax be the main villain and actually be seen throughout the film? Or will he act mainly through his human host in Hector Hammond? I think it would be cooler to see Green Lantern square off with an enemy that looks like a giant dragon, than with an ordinary guy. But perhaps he deals with both in the end.
For comparison purposes, here's a pic of Parallax from his Wikipedia page: